A conventional technique will be described with reference to FIG. 23 and FIG. 24.
FIG. 23 illustrates a pixel configuration of a poly-silicon TFT light emitting display device that uses the conventional technique. Pixels each having an organic light emitting diode (OLED) 207 as a pixel luminous object are arrayed on a display unit in a matrix form. However, FIG. 23 illustrates only one pixel in order for simplification. The pixel 210 is connected to an external drive circuit through a selection line 211, a data line 217, a power supply line 218, and so forth. In the pixel 210, the data line 217 is connected to one end of a canceling capacitor 202 through an input TFT 201. The other end of the canceling capacitor 202 is connected to the gate of a drive TFT 204, one end of a storage capacitor 203, and one end of an AZ switch 205. The other end of the storage capacitor 203 and one end of the drive TFT 204 are commonly connected to the power supply line 218. The other ends of the drive TFT 204 and the AZ switch 205 are commonly connected to one end of an AZB switch 206. The other end of the AZB switch 206 is connected to a common power supply through the OLED 207. Here, the AZ switch 205 and the AZB switch 206 are formed on the TFT, and the gates of these switches are connected to an AZ line 215 and an AZB line 216.
Next, the operation of this conventional example is explained with reference to FIG. 24. FIG. 24 illustrates the drive waveforms of the data line 217, the AZ switch 205, the AZB switch 206, and the input TFT 201, when a display signal is inputted to the pixel. Since the pixel is composed of the p-channel TFTs, the upper (high voltage) side of the drive waveforms in FIG. 24 corresponds to the TFT being OFF, and the lower (low voltage) side corresponds to the TFT being ON.
First, at the timing (1) shown in FIG. 24, the input TFT 201 is turned ON, the AZ switch 205 is turned ON, and the AZB switch 206 is turned OFF. Thereby, the zero (reference) level signal voltage that has been inputted to the data line 217 is inputted to one end of the canceling capacitor 202. At the same time, the voltage across the gate and source of the drive TFT 204 being put into a diode connection by the AZ switch 205 (turned ON) is reset to the voltage of the power supply line 218+Vth. Here, the Vth represents the threshold voltage of the drive TFT 204. When the zero level signal voltage is inputted, this operation automatically brings the pixel into the zero bias such that the gate voltage of the drive TFT 204 becomes just the threshold voltage.
Next, at the timing (2) shown in FIG. 24, the AZ switch 205 is turned OFF, and a signal voltage of a specific analog level is inputted to the data line 217. Thereby, the specific level signal voltage is inputted to one end of the canceling capacitor 202. By this operation, the gate voltage of the drive TFT 204 varies by an additional amount over the specific level of signal, in comparison to the condition at the timing of the automatic zero bias.
Next, at the timing (3) shown in FIG. 24, the input TFT 201 is turned OFF, the AZB switch 206 is turned ON. Thereby, the specific level of signal that has been applied by the input TFT 201 being ON is stored into the canceling capacitor 202. By this operation, the gate of the drive TFT 204 is fixed to a state that the voltage thereof varies by an amount that the specific level of signal is added to the threshold voltage. Further, the signal current (driven by the drive TFT 206) drives the OLED 207 to emit at a brightness corresponding to the specific voltage level of the inputted signal. The conventional technique of this sort is disclosed in detail, for example, in the Digest of Technical Papers, SID 98, pp. 11 through 14, etc.
The conventional technique can not provide an especially satisfactory display quality of animated images or sufficiently suppresses the irregularities of display quality between pixels. The conventional example described with FIG. 23 and FIG. 24 introduces the canceling capacitor 202 and the AZ switch 205, and the AZB switch 206 to absorb the Vth irregularities of the drive TFT 204 into the voltage across the canceling capacitor 202. Thus, the conventional example realizes an analog display with reduced irregularities of brightness in the OLED 207. The conventional example does not concern a satisfactory display quality of animated images. That is, the emission of the OLED 207 starts from the moment of the AZB switch 206 being turned ON, which is illustrated before the timing (3) in FIG. 24, and is continued for virtually one frame, till the moment of the input TFT 201 being turned ON before the timing (1) in the next frame. However, in such a display method, the human eyes are apt to detect the images for continuing two frames so as to visually superimpose them, owing to the afterimage effect of the visual property, which will present unnatural animated images referred as frame retaining.
Although the conventional technique is able to cancel the Vth irregularities of the drive TFT 204 as mentioned above, the characteristic irregularities of the drive TFT 204 are not limited to the Vth irregularities. The conventional technique attains the drive current of the OLED 207 by the current output of the drive TFT 204. This means that the conventional technique also produces brightness unevenness like gain irregularities in each of the pixels, even if the Vth irregularities of the drive TFT 204 can be cancelled (if there are the irregularities of current drive capability due to the irregularities of mobility in the drive TFT 204). Generally, there are large irregularities between individual devices of the TFTs, and it is very difficult to suppress the irregularities between the individual devices, especially when multiple TFTs are packed in a pixel. In case of the low temperature polycrystalline silicon TFT process, for example, the irregularities of mobility are known to appear in about ten percents. Therefore, the conventional technique can not sufficiently suppress the generation of brightness unevenness due to irregularities of display quality between the pixels.